1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to architectures for data/voice services to mobile users, using stratospheric platforms.
2. Description of the Related Art
Stratospheric platforms are being considered for data/voice communication applications. Current proposals envision a mounting of transceivers and antennas on aircraft flying at 20-30 kilometers above the earth which will project beams to cell sites located on the ground.
Copending U.S. patent application Ser. No. 09/588,395, filed Jun. 6, 2000 by D. Chang et al., entitled STRATOSPHERIC PLATFORM BASED MOBILE COMMUNICATIONS ARCHITECTURE, the teachings of which are incorporated herein by reference, addressed the need in the art for a stratospheric platform based communication system offering maximum throughput with the constraints of weight, power and spectrum. In accordance with the teaching of the referenced patent application, communication between users and external networks is facilitated through stratospheric platform and a hub located on the ground. Beamforming is performed at the hub. Signals to and from the user are communicated via directional beams through a phased array antenna on the platform under the directional control of the hub.
Although this system offers a promising solution to the need in the art for a stratospheric platform based communication system and method for projecting beams of varying cell structure to maximize system capacity within the weight, power and bandwidth constraints thereof and thereby optimize the projection of beams with adequate link margin for billable voice and data transmissions, a further need exists in the art for a system and method for tracking the position or location of a user.
For certain applications, the ability to track the position or location of the user would allow for more relaxed platform stability requirements and thereby lower the overall costs of the system. One such application is that of the xe2x80x98third generation mobilexe2x80x99 communications system. As a stratospheric platform application (SPA) this system would provide high data rate communications to a user enabling simultaneous voice, data and entertainment communication. For a lightweight platform payload, both the transmit power and the antenna aperture can be limited. To close a communication link, a larger aperture is needed for the receive end onboard the platform and a larger aperture and more powerful high power amplifiers are needed for the transmit end as well. These requirements are in direct conflict with the objective of a lightweight payload. With beams tracking the mobile users so that the users are always at the antenna peak directivity or within a contour of 1-dB from the peak, the link would have a 3 to 4 dB advantage over the fixed beams.
In addition, since beams would be tracking users, there would be no need to form beams where no users are present except for a new acquisition beam that may scan or a big beam that may be used to zoom in. This arrangement may also save beam forming computations depending on the distribution of users.
In addition, when beams are formed around each user, there may be more opportunities to reuse either the code division multiple access (CDMA) codes or the carrier frequency thereof. This would result in higher system capacity for a limited spectrum.
Knowledge of the user location would also allow for fewer CDMA code handoffs. In a fixed-cell-structure system, when a user crosses a boundary of two cells, CDMA code handoff must happen to avoid interference. With a beam following a user scheme, the user would not have to change his CDMA code unless he gets too close to another user who is using the same CDMA code. A code assignment algorithm is a subject of a copending U.S. patent application Ser. No. 09/735,861 entitled DYNAMIC CELL CDMA CODE ASSIGNMENT SYSTEM AND METHOD filed Dec. 12, 2000, by Ying Feria et al.
The need in the art is addressed by the system and method for user tracking of the present invention. The inventive system is adapted for use in a wireless communication system and creates a plurality of beams within a coverage area. A first beam is directed at a user in a first microcell and a number of additional beams illuminate microcells immediately adjacent the first microcell. The system is equipped with a mechanism for detecting movement of the user from the first microcell to one of the immediately adjacent microcells. On the detection of movement of the user, the system redirects the first beam from the first microcell to a second microcell, the second microcell being one of the adjacent microcells.
In the illustrative embodiment, the system is implemented in a stratospheric platform based communication system including a hub adapted to communicate with a stratospheric platform. A transceiver and a phased array antenna are disposed on the platform to communicate with the hub and with the user. A second antenna is provided on the platform to communicate with the hub. Beamforming and direction are implemented on the hub and communicated to the platform. The user""s position is detected with a global positioning system receiver, by measuring the strength of a signal received from the user, or by other suitable means. On detection of user movement from the first microcell, the beamforming system redirects the beam to follow the user into a second microcell. Additional beams around the user""s microcell are illuminated to facilitate detection of the users movement.